Digital processing and transmission of electrical signals has become commonplace even for basically analog information. Examples range from handheld digital voltmeters to the transition beginning in the 1960s of the public long distance telephone network from analog transmission to pulse code modulation (PCM) digital transmission. Application of digital methods to analog information requires an analog-to-digital (A/D) conversion, and the linearity, resolution, and speed of such conversion depends upon the application. For example, digital voltmeters usually call for A/D conversion with good linearity and resolution (18-bits) but which may be slow (1 Hz); whereas, video applications demand high speed (30 million samples and conversions per second) but tolerate low resolution (8-bits) and poor linearity. Intermediate requirements of 12-bit resolution, good linearity, and 3 Msps (million samples per second) speed appear in applications such as medical imaging with ultrasound, robotic control, high speed data acquisition, process control, radar signal analysis, disk drive head control, vibration analysis, waveform spectral analysis, and so forth. Multichannel information acquisition with arrays of A/D converters leads to another requirement: small aperture jitter so that synchronism of the channels can be maintained.
Well known types of A/D converters include the successive approximations converter which produces a digital output by a succession of trial-and-error steps using a digital-to-analog converter (DAC) and the flash converter which compares an input signal to multiple reference levels simultaneously and outputs a digital version of the closest reference level in a single step. The successive approximations converter provides high resolution and linearity but with low conversion speed, and the flash supplies high speed at the cost of resolution and linearity. Note that a flash converter with n-bit resolution typically has a voltage divider with 2.sup.n taps and 2.sup.n comparators, and this becomes unwieldy for high resolution. See, however, copending U.S. patent application Ser. No. 696,241, filed May 6, 1991 and assigned to the assignee of the present application. A compromise between these two types is the two-step flash A/D converter which uses a first coarse flash conversion to find the most significant bits and then reconstructs an analog signal from first flash output and subtracts this from the input signal to create an error signal from which a second flash conversion finds the least significant bits. Generally see Grebene, Bipolar and MOS Analog Integrated Circuit Design (Wiley-Interscience 1984), page 871. It is desirable that A/D converters combine still higher speed and resolution with lower noise.
Methods of fabrication used for various semiconductor devices include the combination of bipolar transistors with CMOS transistors (BiCMOS), with analog portions of the integrated circuit using mainly bipolar transistors for their low noise and digital portions using mainly CMOS transistors for their high packing density. See for example R.Haken et al, "BiCMOS Processes for Digital and Analog Devices," Semiconductor International 96 (June 1989). However, improved BiCMOS fabrication methods are needed to achieve higher speed and resolution with lower noise on a monolithic circuit.
The present invention provides a monolithic two-step flash A/D converter with high speed and resolution and a BiCMOS method of fabrication applicable to such converters and other integrated circuits.